1. Field of the Invention
The present invention relates to a technology for measuring aspheric surface profile of an optical element such as an aspheric lens.
2. Description of the Related Art
Measurement of aspheric surface profile of an optical element such as a lens or a mirror requires high accuracy and high throughput. Such high accuracy and high throughput measurement of the aspheric surface profile means measurement capable of simultaneously measuring the entire aspheric surface profile. Japanese Patent Laid-Open No. 09-329427 discloses, as a measuring apparatus capable of performing such aspheric surface profile measurement, an interferometer using a null lens. This interferometer is constituted by a Fizeau interferometer or the like provided with the null lens, and projects a reference light (illumination light) having a wavefront corresponding to designed profile of a measurement object aspheric surface. Moreover, the interferometer causes a detection light (reflected light) from the measurement object aspheric surface to interfere with the reference light, measures a difference between a wavefront of the detection light and the wavefront of the reference light and thereby calculates the aspheric surface profile of the measurement object aspheric surface.
Moreover, Japanese Patent Laid-Open No. 2003-050109 discloses, as an aspheric surface measuring apparatus using no null lens, a measuring apparatus using, as a light-receiving sensor, a Shack-Hartmann sensor whose dynamic range for wavefront measurement is wide. This measuring apparatus projects an illumination light that is a spherical wave onto a measurement object aspheric surface through an illumination optical system. Since the measurement object aspheric surface is an aspheric surface, the illumination light is not projected perpendicularly to the measurement object aspheric surface in a region where the measurement object aspheric surface exists, and therefore a ray angle of a detection light reflected by the measurement object aspheric surface is different from that of the illumination light projected onto the measurement object aspheric surface. Consequently, the reflected detection light entering the light-receiving sensor is not collimated, which has a wavefront significantly different from a planer wavefront. Thereby, detection of this wavefront enables calculation of profile of the measurement object aspheric surface.
However, use of the null lens requires production thereof with a lot of time and cost because measurement accuracy depends on production accuracy of the null lens. Moreover, use of the null lens requires provision of null lenses different for each measurement object aspheric surface profile.
On the other hand, when not using the null lens, curvature of the wavefront of the illumination light projected onto the measurement object aspheric surface does not coincide with that of the measurement object aspheric surface. In this case, reflected lights from different reflection points on the measurement object aspheric surface are focused at a same point on the light-receiving sensor, which makes it impossible to specify the reflection points on the measurement object aspheric surface from the reflected lights entering the light-receiving sensor. In addition, the reflected lights entering the light-receiving sensor have angles exceeding a maximum measurable angle of the sensor.
In order to avoid such problems, the measuring apparatus disclosed in Japanese Patent Laid-Open No. 2003-050109 moves the measurement object aspheric surface in an optical axis direction so as to make the curvature of the wavefront of the illumination light and the curvature of the measurement object aspheric surface close to each other. However, when measuring a measurement object aspheric surface whose curvature changes greatly in its radial direction, even though a curvature of the measurement object aspheric surface at a certain point coincides with the curvature of the wavefront of the illumination light, curvatures of the measurement object aspheric surface at other points significantly differ from the curvature of the wavefront of the illumination light. As a result, the above problems cannot be sufficiently solved.